Putting on Moba to share with friends.

Harass with Siphon of Destruction every chance you get. Every time its up just hit the squishiest enemy in your lane. Try to also hit the minions as well to build armor. Throw Creeping Death on friendly minions as they run into the enemy champs, picking the minion who is 2nd or 3rd in line. Never place it on friendly caster minions. Placing it on friendly siege units right before they start taking hits from the tower can help bring down a tower slightly quicker. Play as aggressive as possible early on, but realize that some enemy combinations aren't worth the risk of overextending.

You can get a lot of kills early game with Ignite and once you hit 6 with Children of the Grave + Ignite.

Later on once you have Rylai's and team fights begin you need to be up front. Try to keep a shield up as much as possible. Make sure to Children of the Grave the correct enemy. If all enemies are around the same health I try to ult the enemy AD, but at the same time if a squishy overextends then Ignite + Ult + Q + E should demolish them.

You could go with Ability Power glyphs (blue), and I have in the past, but I think that the magic resist helps Morde harass early game without taking as much damage.

I don't recommend switching out the straight AP quintessences for AP/per level because the straight AP helps you get those early kills.

There's not much you can do differently as far as masteries.

Make sure that after building Force of Nature, when you are building Sunfire Aegis, always buy Giant's Belt before buying Chain Vest. You desperately need the extra health, and it can easily mean the difference in a fight.

The last item is up to you and it just depends on the game and how you're doing. Guardian Angel is a great choice to soak up more damage and for the rebirth. Rabadon's Deathcap for more AP. Alternatively, you could buy a Mejai's Soulstealer right after buying the Sunfire Aegis, it's just 1235gp and buying it early gives you time to build stacks. You could also get a Hextech Gunblade for the spell vamp and active. I tend to get Thornmail if going against more than one heavy AD champ such as Trynd. You could also get Frozen Mallet for the slow and extra health.

Siphon of Destruction is your bread and butter so that's what we level up first. The difference between the first two ranks of this ability is significant. Harass squishy laners with it constantly.

I think that Ignite is a must. It works so well with Children of the Grave. I personally like Ghost as my other summoner spell but I have used Flash in the past and liked it as well. It's up to you.